THE DECIBEL (dB)
Last updated 23 Februrary 2009
The term decibel, or dB, is commonly referred to within the Amateur Radio community. It is a much needed term yet is often misunderstood. Basically, what the term decibel depicts is either a voltage or a power ratio. A related term is dBm which is a power ratio expressed as a level above or below a reference signal. Generally we talk about 0 dBm being equal to 1 milliwatt (mW) of power into a 50 ohm load. The chart below lists the power ratio at various dB levels. You will see that a 10 dB gain is a powewr increase of 10 times. However, a 5 dB increase in power is just over a 3X increase. These two levels may not seem to correlate and they do not. A decibel is a logarithmic function. I have rounded off the power ratios to simplify the numbers. Going beyond one decimal place for our purposes does little good anyway.
|
Decibel
|
Ratio
|
|
0
|
1.0
|
|
1
|
1.26
|
|
2
|
1.59
|
|
3
|
2.0
|
|
4
|
2.5
|
|
5
|
3.2
|
|
6
|
4.0
|
|
7
|
5.0
|
|
8
|
6.3
|
|
9
|
7.9
|
|
10
|
10.0
|
|
11
|
12.6
|
|
12
|
15.9
|
|
13
|
20
|
|
14
|
25.1
|
|
15
|
31.6
|
|
16
|
39.8
|
|
17
|
50.1
|
|
18
|
63.1
|
|
19
|
79.4
|
|
20
|
102
|
|
30
|
103
|
|
40
|
104
|
|
60
|
105
|
|
70
|
106
|
Since calculating power with voltage and resistance being known involves using the formula E2/R the voltage ratio equivalent for a decibel is much less then that for power. Since we don't generally use the voltage ratio as a form of measurement (though it could be used) I have not created a voltage ratio chart. Suffice to say that the ratios would be less with the difference increasing rapidly at the high dB levels.